We know that an asteroid slammed into what is now Mexico’s Yucatan peninsula about 66 million years ago, killing three-quarters of living organisms, including all dinosaurs except for birds. But the first million-year period after this mass extinction has a notoriously poor fossil record, says Tyler Lyson at the Denver Museum of Nature and Science. “I’ve spent 15 years looking for fossils in this interval and have only found a handful of scrappy mammal jaws,” he says.

That changed when Lyson and his colleagues stopped looking for naked bones and started searching for concretions – types of rocks that can form around organic materials, including bone. This turned up a treasure trove of new discoveries in the Corral Bluffs region of Colorado. “We were finding whole skulls, and in some cases skeletons, of mammals, crocodiles, turtles, and then right next to them we were finding plants,” says Lyson.

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The researchers found nearly 1000 vertebrate fossils and over 6000 plant fossils inside the concretions they collected. Dating the rocks allowed them to come up with a detailed chronology of life’s recovery following the asteroid collision.

In the aftermath of the extinction, the most common plants were ferns, and the surviving mammals only weighed half a kilogram. These included Mesodma, a mammal that resembled a rodent, and early relatives of hoofed animals.

At the 700,000-year mark, mammals reached up to 50 kilograms in size. They included a plant-eating, rodent-resembling mammal, Taeniolabis taoensis, and a relative of modern hoofed animals, Eoconodon coryphaeus. This growth spurt coincided with the appearance of the first legume – which Lyson calls the “protein bar moment” – since it probably provided the calories needed to drive this rapid growth.

When the researchers looked back through the climate record, they found that these three bursts of mammal evolution seem to coincide with temperature increases of about 5°C. This suggests that the rise of mammals was helped along by a more tropical climate that enhanced plant growth and hence increased the food supply for animals, says Lyson.

The fossil findings are now “unquestionably the best record of what happened after the [dinosaur] extinction,” says P. David Polly at Indiana University. “They show that the diversity of mammals and plants recovered within about 100,000 years of the event, which is the blink of an eye in geological terms.”

Having such a detailed picture of the way ecosystems recovered after the last extinction event may help us predict what will happen following the sixth mass extinction, which some experts believe is happening now as a result of rapid climate change, says Gregory Webb at the University of Queensland, Australia.

The Corral Bluffs fossils still have many secrets to share, says Lyson. Future studies analysing the skull shape, limb anatomy and inner ear canals of the mammals could provide clues about what they ate, their bite force, whether they were climbing or burrowing animals, and how they moved, he says.